1. Field of the Invention
This invention relates to projection lens systems for use in projection televisions and, in particular, to high performance projection lens systems which can be thermally-stabilized so that the focus of the system does not substantially change between room temperature and the system's operating temperature.
2. Description of the Prior Art
Projection lens systems for CRT projection televisions have undergone continuing development during the past fifteen years or so. As a result, many of today's CRT projection televisions are equipped with fast lens systems which provide wide semi-fields of view.
When the field of view does not exceed a half-angle of about 25.degree., the lens system may consist of just three components and still provide a sufficiently high level of image quality. A typical configuration has a weak aspherical element on the image side of the lens system, followed by a strong positive power element, followed in turn by a strong negative element in close proximity to the CRT. See, for example, Betensky, U.S. Pat. Nos. 4,300,817, 4,348,081, and 4,526,442.
For this configuration, the aspherical first element provides correction primarily for aperture dependent aberrations including spherical aberration, the positive element provides most of the optical power of the system, the relative position of the aperture stop provides correction for astigmatism and lateral color, and the negative element close to the CRT provides correction for field dependent aberrations including the field curvature of the lens.
More generally, lens systems of this type can include more than three elements with the elements being arranged in three lens units, i.e., a "first" or "A" lens unit located on the image side of the lens system of weak power, a "second" or "B" lens unit following the first lens unit of strong positive power, and a "third" or "C" lens unit following the second lens unit of strong negative power.
When the focal length of the lens system is shortened to make the projection television more compact in the case of rear projection televisions or to make the magnification larger in the case of front projection televisions, the field coverage of the lens system must be increased. When the half-angle of view is increased to about 28.degree., a three element form generally cannot provide an appropriately high level of optical performance.
To address this problem, a fourth element or more generally a fourth unit (hereinafter referred to as the "corrector" or "CR" unit) has been added between the strong positive and the strong negative units of the three unit configuration, i.e., between the B and C units. See Betensky, U.S. Pat. No. 4,697892, and Moskovich, U.S. Pat. Nos. 4,682,862, 4,755,028, and 4,776,681. This additional unit usually does not have much optical power; however, it must have an aspherical surface to correct for aperture dependent off-axis aberrations like sagittal oblique spherical and coma.
Color images for projection televisions are normally obtained by combining images from three color CRTs, i.e., a red CRT, a green CRT, and a blue CRT. The phosphors used in commercially available CRTs do not emit light at a single wavelength. In particular, green phosphors have significant sidebands in blue and red. Similar polychromaticity exists for red and blue phosphors, but to a lesser extent.
For many consumer applications, lens systems uncorrected for color can be used, notwithstanding the color spread of the CRTs. For more demanding applications, however, such as high definition television, data displays, or systems which operate at a high magnification, color correction is needed to avoid visible color fringing and/or a loss of image contrast.
A color-corrected projection lens is disclosed in Kreitzer, U.S. Pat. No. 4,900,139. This lens system employs the A, B, C, and CR units discussed above and achieves longitudinal color correction through the use of three lens elements in the B unit which are composed of suitable optical materials and have from the image end of the unit a negative/positive/positive arrangement. Although lens systems constructed in accordance with this patent have worked well in practice, these systems can be susceptible to focus shifts with changes in temperature. Also, the field angles of these systems have been somewhat less than that needed for certain wide angle applications.
Betensky, U.S. Pat. No. 4,801,196, discloses projection lenses having a first lens unit which includes multiple lens elements of weak optical power. As discussed more fully below, in accordance with the invention, it has been determined that the improvement in performance achievable through the use of multiple elements in the first lens unit is limited if those elements are of weak power.
The lens systems of the '196 patent also differ from those of the present invention in that they include a first lens unit which has an overall negative power. In contrast, the lens systems of the present invention have a first lens unit whose overall power is positive.
The lens elements of the '196 patent have highly aspheric surfaces such that their powers when calculated using their on-axis curvatures "may not be representative of the overall power contribution of the lens elements" ('196 patent, column 6, lines 10-11). To deal with this problem, the '196 patent provides curvatures obtained by fitting a spherical surface to the lens elements' aspherical surfaces ('196 patent, column 6, line 9, to column 7, line 51). In contrast, the lens systems of the present invention have lens elements whose on-axis curvatures are representative of the elements' powers.
Using on-axis curvatures, the lens systems of FIGS. 5 and 7 of the '196 patent have first lens elements which are negative followed by second lens elements which are positive, with the first-order, combined power of the first and second lens elements being positive for the FIG. 5 embodiment and negative for the FIG. 7 embodiment. ('196 patent, Tables VII and VIII for the FIG. 5 embodiment and Table V for the FIG. 7 embodiment). Using the sphere fitting approach, the first two lens elements of the FIG. 5 embodiment are again negative/positive but their combined power is now negative. ('196 patent, Tables VII-A and VIII-A). For the FIG. 7 embodiment, the sphere fitting approach gives a negative/negative sequence for the first two lens elements, with a combined power which is again negative. ('196 patent, Table V-A). In contrast, the lens systems of the present invention have a first lens subunit which is negative, a second lens subunit which is positive, and a combined power for the first and second lens subunits which is positive, whether powers are calculated using on-axis curvatures or the sphere fitting approach of the '196 patent (see Table 6).
Projection lens systems employing multiple lens elements in their first lens units are also disclosed in, for example, Moskovich, U.S. Pat. Nos. 4,682,862, 4,776,681 and 4,963,007. As with the '196 patent, these multiple elements have been of weak optical power.